This invention relates to footwear of the type for providing protection against building up of electrostatic charges by dissipation of electrostatic charges through controlled ohmic path leakage between the user's foot and ground.
It has been known for many years to provide footwear which is electrically conductive, exhibiting the capability of conducting electrical charges to ground for various purposes. Thus, such footwear to be worn in certain explosive environments is known as conductive, and may exhibit overall resistance of from zero to about 10.sup.4 ohms. For example, Siers U.S. Pat. No. 2,279,094 discloses conductive footwear of a type to be worn in operating rooms or where explosives are present. Such footwear includes an insole having a conductive metal stiffening member. So also, Berbeco U.S. Pat. No. 4,150,418 discloses a disposable slipper or shoe cover for use in an operating room, and including a strip of conductive material exteriorly of the shoe for providing good electrical conduct with the floor.
On the opposite extreme, shoes to be worn by those exposed to the possibility of electrical exposure, known as insulating, desirably should provide a resistance between the user's foot and ground of from about 10.sup.14 to about 10.sup.19 ohms.
There is an only slightly more conductive range of interest wherein the footwear, termed antistatic, must exhibit very little leakage. Such footwear should have a resistance in the range of 10.sup.9 -10.sup.14 ohms, to prevent static charges from building up.
However, there is a much more practical range of interest, namely footwear exhibiting overall resistance between user's foot and ground in the range of about 10.sup.6 to about 10.sup.8 ohms. Such footwear, termed static dissipating, should be worn by persons in the electronic and computer industries which must observe precautions to avoid and/or to discharge static electricity which may build up upon the body and clothing, since electronic components, such as integrated circuits and other solid state devices, readily can be damaged by electrical discharges and/or potentials which build up on the body and clothing because of static electrical charges. However, footwear having such resistance will not expose the wearer to undue risk of injury from electrical contact, as would conductive footwear. The present invention relates to static dissipating footwear exhibiting resistance in the latter range. Such footwear may also be termed static control footwear.
Various rather cumbersome and sometimes annoying expedients have been proposed in the electronics industry for causing static electricity to bleed off, i.e., to be discharged, from the body and clothing. For example, there have been proposed body grounding heel straps (as in Van Atta et al U.S. Pat. No. 2,586,747), leg straps (as in Legge U.S. Pat. Nos. 2,712,098 and 2,933,651), wrist straps (as in Burke U.S. Pat. No. 3,596,134) and various external devices (as in Price U.S. Pat. No. 2,955,234 which proposed a conductive foil tape for connecting the inside of a shoe to a conductive rubber element exteriorly of the shoe). Straps and the like require attachment when working in an area where protection is to be afforded, but then must be removed subsequently. These devices are annoying as well as uncomfortable and so their use sometimes may be overlooked deliberately.
Although it has been proposed to provide conductive footwear as evidenced by the above-referenced Siers U.S. Pat. No. 2,279,094, footwear which is merely conductive, such as heretofore used for protection of persons in explosive areas, is not at all suitable for use in the electronics industry because of risk of fatal or other serious injury from electrical shock; and existing footwear designs have not been intended for or capable of providing protection against the building up of electrostatic charges through precisely controlled ohmic path leakage, wherein footwear must have resistance precisely controlled in the above noted range of 10.sup.6 to 10.sup.8 ohms.
A fundamental limitation of existing conductive or antistatic footwear designs is that they have not been capable of or well-suited for providing discharge in all modes of usage, from the toe to the heel of the shoe, and irrespective of whether or not the heel is in contact with ground. Alternatively, they may not provide uniform contact of the foot over its entire length within the shoe. The wearer, because of a certain posture or position, such as when seated, may not have the sole and heel both fully in ground contact.
For example, anti-static footwear as disclosed in Dalton U.S. Pat. No. 3,898,538 proposes an electrically resistive panel for connecting the insole to the outsole at only the ball of the foot. Also, for certain body positions, part of the foot may be out of contact with the sock liner. Therefore, a reliable shoe construction intended for providing discharge of electrostatic charges must reliably present a path of discharge over more than just the heel of the shoe, or over more than just the sole or ball of the foot. It must provide its precise range of resistivity interiorly over at least a major portion of the foot and exteriorly over at least a major portion of the length of the shoe from the toe to the heel. Yet, the shoe must be capable of economic manufacture in order to be affordably priced.
Among the several objects of the invention may be noted the provision of improved footwear of electrically conductive type which exhibits resistance in the range useful for providing controlled ohmic path leakage for dissipating electrostatic charges from the body to ground; which provides such dissipation by presenting an ohmic path having resistance in the rang greater than used heretofore for conductive footwear, but less than that used for insulating shoes; which reliably and uniformly exhibits resistance in the range of from about 10.sup.6 to about 10.sup.8 ohms; which provides reliable ground contact for all modes of usage and position of the shoe, by presenting interiorly an area of contact between the foot of the user and the insole over substantially full foot length and width, and exteriorly over the full width and length of the shoe from toe to heel; which is mechanically and reliably constructed to provide a permanent method of construction of the shoe in order to bring about footwear of marked reliability for dissipating static charges; and which during manufacture permits the use of easily obtainable materials without bringing about a major change or modification of the manner of manufacture heretofore used for shoes of the various styles to which the invention is applicable; which utilize novel methodology and principles of construction which can be incorporated into a remarkably wide variety of footwear styles.
Briefly, an electrostatically dissipating shoe of the present invention for providing protection against the building up of electrostatic charges by dissipating such charges through controlled ohmic path leakage between the user's foot and ground. The new shoe includes an insole of controlled resistivity. The insole may be either a separate element of the shoe, or may be formed by the upper of the shoe as folded around for that purpose. An elongated narrow member of flexible slightly electrically conductive material, termed a dissipative strip, is secured to the insole between the insole and the outsole by the use of layers of electrically conductive adhesive above and below the dissipative strip. Further, the shoe is provided with an outsole compounded of a material exhibiting a predetermined resistivity. The outsole has an integrally formed heel. A substantial length of the insole is provided with a plurality of apertures, and electrically conductive adhesive utilized for securing the dissipative strip fills such apertures for providing an electrically conductive path of precisely controlled resistance value from the user's foot, through the dissipative strip, and thence to the outsole including its heel portion for conducting electrostatic charges from the user's foot to ground during all modes of usage of the shoe.
Other objects and features will be in part apparent and in part pointed out hereinbelow.